1. Field of the Invention
The present invention relates to an exposure method, used with a projection exposure apparatus, for fabricating semiconductor devices, image sensing devices (such as charge-coupled devices), liquid crystal displays, thin film magnetic heads or the like. More particularly, it relates to an exposure method suitable for use with a projection exposure apparatus which is equipped with an auto-focusing mechanism for positioning the surface of a photosensitized substrate to cause it to be coincident with an image plane of a projection optical system of the projection exposure apparatus.
2. Description of the Related Art
In photolithographic process for fabricating semiconductor devices or other products, a projection exposure apparatus (such as a stepper) is commonly used in which an image of a pattern on a mask or reticle is transferred through a projection optical system onto each of a plurality of shot areas defined on a photoresist-coated wafer (or other substrate such as a glass plate). In a projection exposure apparatus, the wafer may be held against a top surface of a wafer holder utilizing vacuum, and the top surface of the wafer may have a relief pattern of concentric circles. In addition, such a projection exposure apparatus may include an auto-focusing mechanism comprising an auto-focus sensor system and a stage mechanism. The auto-focus sensor system detects a focusing position (or vertical position) of the surface of the wafer at a measurement point which may be selected, for example, to be at the center of an exposure field of the projection optical system. The stage mechanism uses the detection results from the auto-focus sensor system so as to position the surface of the wafer in each shot area to locate the surface close to and within a range of focal depth about an image plane of the projection optical system.
In a prior art auto-focusing mechanism of this type, calibration is achieved such that the auto-focus sensor system provides a detection signal at a predetermined reference level when the surface of the wafer is coincident with the image plane of the projection optical system. Further, in order to maintain focusing during an exposure operation by an auto-focus technique, a servo-control technique is used to control the focusing position of the wafer through the stage mechanism such that the detection signal from the auto-focus sensor system remains substantially at the reference level.
As described, in this prior art auto-focusing system, the focusing position of the wafer at a preselected measurement point in the exposure field is caused to be coincident with the image plane whose position is known. Thus, if there is a protuberance of the surface of the wafer at a position near the measurement point in the shot area to be exposed, which may occur due to the intrusion of foreign matter, such as a residue of photoresist or a dust particle, between the bottom surface of the wafer and the top surface of the wafer holder, then only the region of the protuberance may be made coincident with the image plane with the result that the remaining large portion of that shot area is positioned unacceptably far from the image plane. Consequently, the chip pattern formed in that shot area will be defective. However, until now, it has been impossible to detect any such defective chip patterns caused by foreign matter before checking the integrity of chip patterns on wafers for after the entire fabrication process has been completed.
If such foreign matter can be detected, useless exposure operations may be eliminated. Specifically, if large foreign matter is detected in a particular shot area during the exposure process for the first level layer on the wafer, exposure operations can be omitted for that shot area during the exposure processes for the second and any subsequent level layers.
Nevertheless, adding a detection device specifically designed for foreign matter detection purpose to the projection exposure apparatus-would result in a complication of the arrangement of the wafer stage and hence in an increase in the manufacturing costs.
In view of the foregoing, it is an object of the present invention to provide an exposure method which is capable of detecting foreign matter existing on the bottom of a wafer without the need for any special detection device.
According to a first aspect of the present invention, there is provided an exposure method in which positioning of a substrate is performed by means of a substrate stage and in which a mask pattern is sequentially projected onto a plurality of shot areas on the substrate through a projection optical system, the method comprising: a first step including causing a first shot area on the substrate to be set in an exposure field of the projection optical system, causing the surface of the substrate and an image plane of the projection optical system to be coincident with each other at a predetermined measurement point in the exposure field so as to achieve focused condition, and transferring the mask pattern onto the first shot area by projection exposure; a second step including moving the substrate along a predetermined running plane by means of the substrate stage so as to cause a second shot area on the substrate to be set in the exposure field of the projection optical system, adjusting vertical position of the substrate by means of the substrate stage so as to compensate for any offset between the running plane and the image plane of the projection optical system, and measuring and storing a defocus amount between the surface of the substrate and the image plane of the projection optical system at the predetermined measurement point; and a third step including causing the surface of the substrate and the image plane of the projection optical system to be coincident with each other at the predetermined measurement point so as to achieve focused condition, and transferring the mask pattern onto the second shot area by projection exposure.
This exposure method may preferably further comprise the steps of: repeating the second step and the third step for a plurality of shot areas on the substrate so as to obtain a plurality of defocus amounts; and detecting from the plurality of defocus amounts the existence of any foreign matter existing between the substrate and the substrate stage.
Further, this exposure method may further comprise the steps of: repeating the second step and the third step so as to obtain a plurality of defocus amounts; obtaining from the plurality of defocus amounts a distribution of tilt angles on the surface of the substrate; determining, by calculation, from the distribution thus obtained, positions and size of any foreign matter existing between the substrate and the substrate stage; and displaying the positions and size of foreign matter thus determined on a display.
According to a second aspect of the present invention, there is provided an exposure method in which positioning of a substrate is performed by means of a substrate stage and in which a mask pattern is sequentially projected onto a plurality of shot areas on the substrate through a projection optical system, the method comprising: a first step including causing a first shot area on the substrate to be set in an exposure field of the projection optical system, adjusting vertical position and tilt angle of the surface of the substrate to those of an image plane of the projection optical system, and transferring the mask pattern onto the first shot area by projection exposure; a second step including moving the substrate along a pre-determined running plane by means of the substrate stage with tilt angle of the substrate being kept unchanged so as to cause a second shot area on the substrate to be set in the exposure field, measuring and storing tilt angle of the surface of the substrate relative to the image plane in the exposure field, and thereafter adjusting vertical position and tilt angle of the surface of the substrate to those of the image plane and transferring the mask pattern onto the second shot area by projection exposure; a step of repeating the second step for a plurality of shot areas on the substrate so as to obtain a distribution of a plurality of tilt angles and using the distribution so as to detect any foreign matter existing between the substrate and the substrate stage.
According to a third aspect of the present invention, there is provided an exposure method in which positioning of a substrate is performed by means of a substrate stage and in which a mask pattern is sequentially projected onto a plurality of shot areas on the substrate through a projection optical system, the method comprising: a first step including causing a first shot area on the substrate to be set in an exposure field of the projection optical system, adjusting a vertical position of the surface of the substrate to that of an image plane of the projection optical system, storing tilt amount of the surface of the substrate relative to the image plane, and transferring the mask pattern onto the first shot area by projection exposure; a second step including moving the substrate along a predetermined running plane by means of the substrate stage with tilt of the substrate being kept unchanged so as to cause a second shot area on the substrate to be set in the exposure field of the projection optical system, and measuring and storing a defocus amount between the surface of the substrate and the image plane of the projection optical system at a predetermined measurement point; and a third step including making the surface of the substrate and the image plane of the projection optical system coincident with each other at the predetermined measurement point so as to achieve focusing condition, and transferring the mask pattern onto the second shot area by projection exposure.
In this exposure method, the tilt amount may be determined, by calculation, from i) tilt angle of the shot area relative to the image plane and ii) spacing between the shot areas.
This exposure method may preferably further comprise the steps of: repeating the first step, the second step and the third step for a plurality of shot areas on the substrate so as to obtain a plurality of defocus amounts; and detecting from the plurality of defocus amounts the existence of any foreign matter between the substrate and the substrate stage.
Moreover, this exposure method may further comprise the steps of: repeating the first step, the second step and the third step so as to obtain a plurality of tilt amounts and a plurality of defocus amounts; obtaining, from the plurality of tilt amounts and the plurality of defocus amounts, distributions of tilt amounts and defocus amounts on the surface of the substrate; determining, by calculation, from the distributions thus obtained, positions and size of any foreign matter existing between the substrate and the substrate stage; and displaying the positions and size of foreign matter thus determined on a display.
In the exposure method according to the first aspect of the present invention described above, for example, in the first step when the first shot area on the substrate is exposed, a point P in the first shot area is caused to be coincident with the image plane, as shown in FIG. 3(a). Then in the second step, in order to expose the second shot area on the substrate, the substrate stage is moved along the predetermined running plane to cause the second shot area to be set in the exposure field, as shown in FIGS. 3(a) and 3(b). This movement of the substrate stage produces a variation xcex4 (≈L*xcex8) in the focusing position of the substrate, where xcex8 stands for the known value of the tilt angle of the running plane relative to the image plane and L stands for the distance covered by the movement of the substrate stage.
Accordingly, the focusing position is adjusted so as to compensate for the variation xcex4, by operating the substrate stage, as shown in FIG. 3(c). This causes a point Q in the second shot area to be coincident with the image plane so as to achieve focused condition provided that the substrate is flat. However, if there is foreign matter between the substrate and the substrate stage as shown in FIG. 3(d), a non-zero defocus amount xcex94Z is measured between the point Q and the image plane. Therefore, any foreign matter existing on the bottom surface of the substrate may be detected by using an ordinary auto-focus sensor system to measure the defocus amount xcex94Z.
In this method, the second step and the third step may be repeated so as to obtain a plurality of defocus amounts, from which a distribution of tilt angles on the surface of the substrate may be obtained. The distribution of the tilt angles indicates a peak at a position where a large foreign matter exists on the bottom surface of the substrate. Thus, positions and size of any foreign matter on the bottom surface of the substrate may be determined, by calculation; from the distribution of the tilt angles, and the positions and the sizes of foreign matter thus determined may be displayed on a display.
In the exposure method according to the second aspect of the present invention described above, for example, in the first step the first shot area on the substrate is set parallel to the image plane, and in the second step the second shot area is set in the exposure field with the tilt angle of the substrate being kept unchanged. If there is foreign matter on the bottom surface of the substrate at a position corresponding to or near the second shot area, a certain tilt angle of the surface of the second shot area relative to the image plane is detected. By repeating the second step, a distribution of the tilt angles of the substrate due to foreign matter is detected. Thus, positions and size of any foreign matter may be determined from the distribution of the tilt angles. Since the tilt angles of the surface of the substrate may be detected by means of an ordinary levelling sensor, it is unnecessary to use any special detection device designed for foreign matter detection purpose.
In the exposure method according to the third aspect of the present invention described above, for example, in the first step when the first shot area on the substrate is exposed, a point P in the first shot area is caused to be coincident with the image plane and the tilt amount of the surface of the shot area relative to the image plane is measured, as shown in FIG. 3(a). Then in the second step, in order to expose the second shot area on the substrate, the substrate stage is moved along the predetermined running plane to cause the second shot area to be set in the exposure field, as shown in FIGS. 3(a) and 3(b). This movement of the substrate stage produces a variation xcex4 (≈L*xcex8) in the focusing position of the substrate, where xcex8 stands for the known value of the tilt angle of the running plane relative to the image plane and L stands for the distance covered by the movement of the substrate stage. Accordingly, the variation xcex4 is determined from i) the tilt amount xcex8 which has been measured for the first shot area and ii) the distance L covered by the movement of the substrate stage (or the step pitch) and stored. Then, the defocus amount (xcex94Zxe2x80x2) at the point Q is measured and a calculation xcex94Z=xcex94Zxe2x80x2xe2x88x92xcex4 is performed. If the substrate is flat, we obtain xcex94Z=0. However, if foreign matter exists between the substrate and the substrate stage as shown in FIG. 3(d), a non-zero defocus amount xcex94Z is measured between the point Q and the image plane, so that it is determined that foreign matter may exist.
In this method, the first step, the second step and the third step may be repeated so as to obtain a plurality of defocus amounts, from which a distribution of the defocus amounts on the surface of the substrate may be obtained. Therefore, positions and size of any foreign matter on the bottom surface of the substrate may be determined, by calculation, from the distribution of the defocus amounts, and the positions and the sizes of foreign matter thus determined may be displayed on a display.